Make Property a monad/functor

[grmble]

If Property were a monad, event handling code would be simpler and more powerful.

EventDecoder might become unnecessary.

[grmble]

This would make the code simpler, but it would be a performance problem. Ideally, the decoding functions in a VirtualDom on-Node should be comparable. That is a hard problem, so object identity is compared. If the functions remain the same javascript objects between render passes, expensive DOM updates can be skipped. Functions are only ever the same object if they are compiled down to a module-level function (not a curried function!)

Composing Properties after their creation would defeat this.

The path forward is to make EventDecoder a functor/monad, this makes it easy to define a module-level helper function that is used to create the event handling Property.